Implantable cardioverter-defibrillator (ICD) art has long distinguished ventricular tachyarrhythmias by rate and type. Ventricular tachycardias (VTs), which are typically identified by their rate, can be further differentiated by their ECG configuration as either monomorphic or polymorphic. Arrhythmias with rates above an upper VT range, and up to a predetermined rate limit, are often termed flutter waves. Ventricular tachyarrhythmias at rates higher than the predetermined rate limit are classified as ventricular fibrillation (VF).
To treat each type of arrhythmia with an appropriate therapy, ICDs have been equipped with “tiered therapies”. ICDs generally differentiate arrhythmias by rates, with programmable therapies to treat a respective type of detected arrhythmia(s). In such devices, arrhythmias such as VT are treated by delivering a series of low-power pacing pulses to the heart at a relatively high rate. This therapy is often referred to as anti-tachyarrhythmia pacing therapy (ATP). In contrast, arrhythmias such as VF, cannot be pace-terminated and are therefore treated using a more aggressive shock therapy. For example, many ICDs may be programmed to first treat a VT with low-power ATP and then, if the VT is not terminated by ATP or accelerates to ventricular flutter or fibrillation, deliver one or more high-power cardioversion or defibrillation shocks.
The rates of VT and VF overlap. Since VF poses the greater hazard to the patient, the rate used to discriminate VT and VF is selected to rarely classify VF as VT. For this reason, VTs having rates above a predetermined rate are often treated by high-energy shocks for VF when, in fact, they are monomorphic VTs that might be successfully terminated by low-energy ATP therapy. As a result, some patients must unnecessarily endure the pain of receiving a high-voltage shock delivery when painless ATP could have successfully terminated the rhythm.
What is needed therefore is a method and apparatus for discriminating arrhythmia events to reduce the utilization of high-voltage shocks.